Great progress has been made in silicon IC (integrated circuit) technology. However, it has been a trend to increase the packaging density of devices. The large integration of semiconductor ICs has been accomplished by a reduction in individual device size. With this reduction of device size, many challenges arise in the manufacture of the ICs. As an example, the reduction in a DRAM cell size results in a decrease in storage capacitance leading to reliability drawbacks, such as a lower signal to noise ratio and other undesirable signal problems.
Each device requires interconnections for exchanging electrical signals from one device to another device. Therefore, ICs includes a metallization in the form of a pattern that is extends from the surface of a substrate. In the technology, the metallization extends through an opening in an isolation material and makes ohmic contact with the substrate with the substrate. As the numbers of the components in the ICs is increased, the technology towards the use of multiple level of interconnection with an isolation structure between adjacent levels. The formation of the interconnection is now even more important and the interconnection for connecting elements between semiconductor devices is becoming more critical as the ICs toward multi-level interconnections.
Typically, the interconnection or other electrically conducting elements is widely used for providing specific conducting paths in an electrical circuit. In the process of forming the electrical conducting structure over an underlying layer, it is important that the surface of the underlying layer be free of any contamination, such as moisture, particles or oxides. Further, the surface of the underlying layer must be planar for subsequent layer deposition.
Tungsten is one of the common materials used in the formation of an electrical conducting structure. Before forming a tungsten layer 10 over a underlying layer, a composition layer consisting of Ti layer 6 and TiN layer 8 must be formed along the surface of the underlying layer to act as a barrier layer. Typically, the tungsten layer 10 and the barrier layer are filled in an opening of an isolation layer 4 formed over a substrate 2, as shown in FIG. 1. A hole 12 may be formed in the tungsten layer 10 due to the step coverage relating to the deposition characteristic. A planarization process is typically employed after the tungsten layer 10 is formed to obtain smoother surface. In general, there are two ways to obtain the aforementioned purpose.
One of the methods for planrization is to etch back the tungsten layer 10 by using etching technique. The benefit of the methods is that the cost is lower than the others. However, the so-called "key-hole" phenomenon will occur after the etching, as shown in FIG. 2. A "key hole" shape opening 14 is generated by the over etching and the etching selectivity between the tungsten and the barrier layer. A number of tungsten residues 16 are remained on the surface of the substrate. This effect creates undesired tungsten particles on the surface of the tungsten layer. In addition, the etching process also causes the substrate damage.
Another way is to introduce the chemical mechanical polishing (CMP) for polishing the entire surface of the substrate. As known in the art, the CMP exhibits high selectivity between oxide and metal. An effect relating to the polishing rate that is called "dishing effect" is occurred during the polishing, as shown in FIG. 3. The phenomenon is attributed to the high polishing selectivity for the tungsten material. The polishing rate of the tungsten is higher than that of the adjacent material. Thus, hollow portions 18 is generated on the surface of the tungsten, which is referred to the "dishing effect". Although the CMP exhibits high yield accompany with high cost, there are other drawbacks for the CMP such as the alignment mark damage, scratch, erosion and the metal contamination. The oxide may be loss during the polishing.
What is required is a method of forming a tungsten layer free from particle contamination, dishing effect and so on.